(1) Field of the Invention
The present invention relates to a heterojunction bipolar transistor which has been widely used for a communication transmitting high power amplifier.
(2) Description of the Related Art
Conventionally, a compound semiconductor device such as a Field-Effect Transistor (hereafter referred to as FET) and a Heterojunction Bipolar Transistor (hereafter referred to as HBT) have been used for a communication transmitting high power amplifier that is one of the components of a cellular phone, and the like.
FIG. 1 is a cross-section diagram showing a device structure of a typical conventional HBT (e.g. refer to Japanese Laid-Open Patent Publication No. 2001-326231).
As shown in FIG. 1, in the conventional HBT, by a crystal growth using a MOCVD method (Metal Organic Chemical Vapor Depoint method) or a MBE method (Molecular-Beam Epitaxial method), the following layers are sequentially laminated on a semi-insulating GaAs substrate 500, the layers being a n-type GaAs subcollector layer 501 with a thickness of 6000 Å formed by doping n-type impurities at a concentration of 4×1018 cm−3, a n-type GaAs collector layer 502 with a thickness of 6000 Å formed by doping n-type impurities at a concentration of 1×1016 cm−3, a p-type GaAs base layer 503, a n-type InGaP second emitter layer 504, a n-type GaAs first emitter layer 505, and a n-type InGaAs emitter contact layer 506. Also, an emitter electrode 507, a base electrode 508 and a collector electrode 509 are respectively formed on the emitter contact layer 506, the base layer 503 and the subcollector layer 501 that are formed by process technologies such as lithography, etching and evaporation.
Next, an evaluation result on typical electric characteristics of a conventional HBT having the structure mentioned above is explained.
FIG. 2A is a “Gummel plot” that indicates dependencies of a collector current Ic and a base current Ib on a voltage Vbe in the case where the base and the collector are connected to a common electrode. FIG. 2B is a diagram indicating characteristics of a collector-emitter voltage Vce and a collector current Ic with grounded emitter. In FIG. 2A, a solid line indicates Ic and a dotted line indicates Ib. Also, in FIG. 2B, the Ic-Vce characteristic is seen at the Ib which is different points at 0, Ibm/10, Ibm/2 and Ibm, and Ibm indicates the maximum value of Ib in FIG. 2B.
FIG. 2B shows that, when the Vce increases and reaches a certain value, Ic suddenly increases and a HBT breaks down. This phenomenon that Ic suddenly increases at a certain Vce is called an avalanche breakdown. The avalanche breakdown is a phenomenon of generating an electron and a hole one after another by electrons running rapidly in the collector layer to collide with ambient atoms when a state of reserve bias between the collector and the base is intensified and gradually the intensity of the electric field is increased to the extent. It is also called impact ionization. Here, a current at which the avalanche breakdown occurs is generally expressed as in the following (1) equation by applying αn and αp respectively to impact ionization coefficients of an electron and a hole and applying Jn and Jp respectively to current densities of the electron and the hole.αnJn+αpJp  (1)
Such avalanche breakdown occurs resulting from a presence of carriers of the electron, the hole and the like, and of an intensity of the electric field in a channel. Accordingly, the greater the number of carriers increases, the more likely the avalanche breakdown occurs. In addition, the stronger the intensity of the electric field is, the more likely the avalanche breakdown occurs. In FIG. 2B, the former is seen from that Vce at which the avalanche breakdown occurs is the minimum at Ib=Ibm when Ic is the maximum. In FIG. 2B, the latter is seen from that the avalanche breakdown occurs when the intensity of the electric field reaches the intensity of a critical electric field (4×105V/cm) even at Ib=0 where there are no carriers.
By the way, in recent years, there has been a demand for the HBT to have great characteristics of high power outputs, high gains and low distortions. For example, the high power output is especially required in the case where the HBT is commercially used as a communication transmitting high power amplifier for a cellular phone of a GSM system but not of the conventional CDMA system.
However, the conventional HBT as shown in FIG. 2B has a problem that a breakdown is likely to occur as Ic increases. In other words, a breakdown voltage is lowered on high power output.
In here, a state inside a device when the conventional HBT breaks down is explained with reference to FIGS. 3A and 3B, and FIGS. 4A and 4B.
FIG. 3A and FIG. 4A are diagrams showing a concentration of donors ionized in positive (hereafter referred to as set concentration) and a concentration of electrons that are negative. FIG. 3B and FIG. 4B are diagrams showing an intensity of an electric field (absolute value). In FIG. 3A and FIG. 4A, the horizontal axis indicates a distance from the surface of the second emitter layer 504 and the ordinate axis indicates the concentration. In FIG. 3B and FIG. 4B, the horizontal axis indicates a distance from the surface of the second emitter layer 504 and the ordinate axis indicates the intensity of the electric field. Also, FIGS. 3A and 3B are the cases where Ic is low, that is, where Ib is Ibm/10 in FIG. 2B. FIGS. 4A and 4B are the cases where Ic is high, that is, where Ib is Ibm in FIG. 2B.
FIG. 3A shows that, when current is low, the set concentration is higher than the concentration of electrons in the collector layer 502 and that inside the collector layer 502 is charged in positive. In here, although it is not shown in the diagram, there is a thin layer of an acceptor ionized and charged in negative on the side of the collector layer 502 of the base layer 503. The negative charges in the surface of the base layer 503 are proportional to the positive charges in the collector layer 502.
FIG. 3B shows that, when current is low, a high electric field equivalent to an intensity of a critical electric field is generated at the interface between the base layer 503 and the collector layer 502 and that an avalanche breakdown occurs. In here, a gradient of the electric field indicates the set concentration. The gradient gets steeper as the set concentration gets higher so that the avalanche breakdown occurs at lower Vce. In addition, the value obtained by integrating the intensity of the electric field, that is, an area, indicates a voltage applied between the base and the collector. Therefore, the larger the area at the time when the avalanche breakdown occurs is, the higher the breakdown voltage is.
FIG. 4A shows that, when current is high, the set concentration is lower than the concentration of electrons in the collector layer 502 and that inside the collector layer 502 is charged in negative. In here, although it is not shown in the diagram, there is a layer charged in positive on the side of the collector layer 502 of the subcollector layer 501. The positive charges in the surface of the subcollector layer 501 are proportional to the negative charges in the collector layer 502.
FIG. 4B shows that, when current is high, the maximum electric field is generated at an interface between the subcollector layer 501 and the collector layer 502 and that the avalanche breakdown occurs (Kirk effect). At this time, the concentration of electrons in the subcollector layer 501 is high and the avalanche breakdown is likely to occur so that the maximum intensity of the electric field is lower than the intensity of the critical electric field. Note that, the phenomenon is explained in detail in the following reference book: William Liu, “Fundamentals of III–V Devices”, pp. 190.
From the above explanations, it is found that, when Ic is high, the breakdown of HBT results from a generation of the maximum electric field on an interface between the subcollector layer and the collector layer.